This invention relates in general to vehicle brake systems. In particular, this invention relates to an electrohydraulic vehicle brake system with an electromechanical actuator for actuating the brake system.
Electromechanical actuators have already been used for some time in vehicle brake systems, for example, for realising an electrical parking brake function (EPB). In the case of electromechanical brake systems (EMB), they replace the conventional hydraulic cylinders at the wheel brakes.
Owing to technical advances, the efficiency of the electromechanical actuators has continually increased. It was therefore considered to use such actuators also for implementing modern driving dynamics control systems. Such control systems include an antilock braking system (ABS), a traction control system (TCS) and an electronic stability program (ESP), also referred to as vehicle stability control (VSC).
WO 2006/111393 A teaches an electrohydraulic brake system having a highly dynamic electromechanical actuator which performs the pressure modulation in the driving dynamics control operation. The electromechanical actuator described in WO 2006/111393 A is provided to act directly on a master cylinder of the brake system.
Owing to the high dynamics of the electromechanical actuator, the hydraulic components of the brake system known from WO 2006/111393 A can be reduced to a single 2/2-way valve per wheel brake. The valve associated with a particular wheel brake is opened to build up or reduce hydraulic pressure at the wheel brake and remains closed during a pressure maintaining phase.
To realise wheel-individual pressure modulations, the valves are then activated individually or in groups in time multiplex operation. In order to be able to carry out a safe driving dynamics control in the time multiplex operation, use is made in the electromechanical actuator of an electric motor which can change its position in a precise manner in a few milliseconds. Nevertheless, there are situations in which the time multiplex operation, due to the system, reaches its limits and therefore optimal driving dynamics control is no longer possible. Thus, in the context of a traction control, there may be situations in which different pressure requirements exist for two wheel brakes and the delay time, resulting from the time multiplex operation, for the setting of the different pressures has disadvantages for the traction control.